In the exploration for petroleum and recovery of hydrocarbons from underground reservoirs, wells are drilled using drilling fluids. The drilling fluids are usually a heavy fluid or slurry and act, among other purposes, as a lubricant for the drill-bit when boring, for the removal of heat and drill cuttings from the bottom of the well, for the chemical balance of the well, and for overcoming hydrostatic pressures in the well. Different drilling fluids can be used and are usually classified as oil based fluids, water based fluid or emulsions. Drilling fluids may also contain high specific density minerals such as barite and include entrained minerals such as heavy metals.
The drilling fluids are circulated from the surface, through the drill string to the drill bit and return to the surface. On their return to the surface, the drilling fluids will include cuttings comprising sand, clay, petroleum such as bitumen or heavy oil, and other chemicals. The drilling fluid and cuttings are processed through a series of shale shakers, desanders, desilters, centrifuges, and other known devices to remove the solids, so that the drilling fluid is separated from the cuttings for reuse of the fluid in the borehole. The cuttings are removed for disposal to a waste pit or landfill site.
Drill cuttings from drilling oil and gas wells generally are composed of sand, mudstone, petroleum such as bitumen and/or heavy oil, water, chemicals, and residual drilling fluid. Once separated from the drilling fluid, these cuttings are stabilized to transport them to a landfill site. The different drilling fluids and their components will contaminate the cuttings and therefore also create a number of environmental issues for disposal of the cuttings.
A number of methods have been developed to dispose of drilling cuttings.
One proposed method for preparing the cuttings for disposal is set out in WO2007/102743. It discloses mixing the drill cuttings with cement to form a set or hardened product for disposal. This product prevents leaching of components into the environment. Another process to treat liquid waste for disposal adds amorphous silica such as rice hull ash to the liquid waste to form a solid or semi-solid mass to stabilize the liquid and increase the ease of transport and containment of the liquid and its contaminants. A further process uses thermal treatments to dry drill cuttings so they can be handled more easily.
U.S. Pat. No. 6,838,485 adds an emulsifier to the cuttings mixture to transform the free hydrocarbons in the cuttings into an emulsion. The emulsion is then treated with an encapsulating material to encapsulate the emulsified hydrocarbons. The encapsulating material surrounds the emulsified hydrocarbon droplets and solidifies. A preferred encapsulating material is silicate solution. The mixture of drill cuttings and encapsulated hydrocarbons is then released into marine waters where it disperses.
U.S. Pat. No. 7,059,805 discloses a process where drilling cuttings are mixed with dehydrated limestone coke ash in the presence of water. The drilling fluid residue in the cuttings becomes entrapped into a non-leaching solid. Following treatment of the drill cuttings with the dehydrated limestone coke ash, the resulting product can be used as a road or work area base.
One common method for treating drill cuttings for disposal in landfill sites is to mix them with sawdust. However, it requires mix ratios of at least 50/50 by volume with sawdust. This process adds costs to the hydrocarbon recovery process including the costs for the sawdust and associated cost of transporting the sawdust to the well site, and the cost of disposal of the total waste which is more than double the total volume of waste by mixing the sawdust and cuttings.
A further issue with these processes to dispose of the cuttings is that the petroleum products, such as bitumen or heavy oil, in the cuttings is not recovered. This petroleum is being disposed with the cuttings and therefore is lost revenue as well as a contaminant.
There is therefore a need for a process which uses traditional equipment but is less expensive to transport and dispose of the cuttings, does not significantly increase the volume of the cuttings, and which may also remove/recover at least a portion of the bitumen and heavy oil from the cuttings prior to disposal.